Doubling seems to be one of the most common errors with alot of varieties out there that make your head spin. I recently show Daniel a 1968D Kennedy Half that I was sure was a Double Die. Just learning about mint errors this year, he quickly corrected me by saying what I had was a " Mechanical Doubling" which are worth nothing in the Numismatic Community. Here's a good website that goes into more detail than found here at this site : http://www.doubleddie.com/144822.html